The present invention relates generally to the refining of lignocellulose material, such as wood chips, in a pressurized environment, and more particularly, to a multi-stage method for refining lignocellulose material, and an apparatus for carrying out the method, wherein the lignocellulose material is refined at pressure in at least an upstream stage refining zone and a downstream stage refining zone with the lignocellulose material being pneumatically transported between stages by the steam generated in the upstream stage refining zone. The method may be advantageously carried out by using disc-type refiners and, most advantageously, utilizing disc-type refiners incorporating high speed shaft driven ribbon feeders.
It is well known in the art to stage, that is to place in series, a plurality of refining zones, typically two, to produce mechanical or chemi-mechanical or thermomechanical pulp from lignocellulose material, most commonly wood chips. In such conventional systems, it is the common practice to pass the lignocellulose material at a high consistency through a first pressurized refining zone to produce a semi-refined pulp entrained in steam generated within the first refining zone. The semi-refined pulp entrained in the steam generated in the first refining zone is passed directly to a steam separating device, most commonly a cyclone, wherein the steam is separated from the semi-refined pulp and the semi-refined pulp is then conveyed, most typically by a screw feeder, to a second stage refining zone. The semi-refined pulp is further refined in the second refining zone to produce a fully refined pulp again entrained in steam generated within the refining zone. The fully refined pulp and steam mixture generated in the second refining zone is discharged directly to a second steam separating cyclone wherein the steam is separated from the pulp and the fully refined pulp recovered.
Typically, such multi-stage, high consistency refining is carried out utilizing in combination a first stage refiner, followed by a steam separating cyclone, followed by pulp conveyor means, followed by a second stage refiner. One such system is shown in U.S. Pat. No. 3,661,328 wherein the wood chips to be refined are first pretreated and then passed to a first stage refiner comprising a pressurized double rotating disc refiner wherein the chips are initially refined to produce a partially refined pulp. The partially refined pulp is conveyed in steam generated in the pressurizing refiner zone within the double rotating disc refiner directly to a steam separating cyclone wherein the partially refined pulp is separated from the steam and recovered. The partially refined pulp is then fed from the discharge of the steam separating cyclone by a screw conveyor directly to a second refiner comprising an atmospheric double rotating disc refiner.
Another method and apparatus for the multi-stage, high consistency refining of wood chips and other lignocellulose material through staged independent refiners is disclosed in U.S. Pat. No. 4,298,425. As shown therein, the wood chips are first subjected to a grinding operation in the refining zone of a pressurized rotating disc type defibrator to produce a partially refined pulp consisting of a flocculent mass of initially separated and freed fibers entrained in steam generated within the first stage refiner, such steam typically having a temperature of 110.degree. to 140.degree. C. The partially refined pulp is conveyed in the steam generated within the first stage refiner directly from the discharge of the first stage refiner to a steam separating cyclone wherein the partially refined pulp is separated from the steam and the steam is recovered. The separated partially refined pulp is then passed from the discharge of the steam recovery cyclone through a screw conveyor to a second stage refiner which again is a rotating disc defibrator wherein the pulp material is further refined to product the final product pulp.
Such conventional multi-stage, high consistency refining of lignocellulose material may also be carried out using a single machine incorporating two refining zones such as shown in U.S. Pat. No. 4,700,900. As disclosed therein, the high consistency refining is accomplished using a refiner which has two virtually identical but separate refining zones defined in a single machine. The wood chips to be processed are first fed by means of a screw conveyor to the first stage refining zone and subjected to defibrating therein to form a partially refined pulp entrained in steam generated within the refining zone. The partially refined pulp entrained in the steam generated in the first refining zone is discharged therefrom directly to a steam separating cyclone disposed interstage between the first stage refining zone and the second stage refining zone. The partially refined pulp is separated from the steam generated within the first refining zone and discharged from the steam separating cyclone directly to a screw conveyor which feeds the partially refined pulp to the second stage refining zone within the machine wherein the partially refined pulp is further refined to produce the final fully refined pulp product.
It would be advantageous if the partially refined pulp could be discharged from the first stage of the refining zone and conveyed in the steam generated within the first refining zone directly into a ribbon feeder for conveying the partially refined pulp to second refining zone and separating the steam therefrom without the use of an interstage cyclone. Heretofore, the direct transfer of partially refined pulp from a first refining stage to a second refining stage has been limited to low consistency pulping operations wherein steam is not generated within the refining zone. In low consistency pulping systems, that is systems wherein the pulp is processed in an aqueous slurry having a solids content of less than about 4% by weight, steam is not generated in the refining step due to the fact that there is sufficient liquid in the aqueous pulp slurry to absorb the heat generated during the refining step without the formation of steam. For example, as shown in U.S. Pat. Nos. 2,864,562 and 3,323,731, two pairs of grinding discs disposed within the same machine are operated in series with the outlet of the first refining zone connected by a substantially U-shaped conduit directly to the inlet of the second refining zone. After the pulp fed to the first refining zone between the first set of grinding discs is subjected to a grinding treatment therein, the partially refined pulp resulting therefrom is conveyed directly to the inlet to the second refining zone between the second pair of grinding discs wherein further grinding treatment is carried out to produce the final product pulp.
It is an object of the present invention to provide a high consistency pressurized multi-stage refining method, and an apparatus for carrying out the method, wherein the lignocellulose material is refined under high consistency pressurized conditions in at least an upstream refining zone and a downstream refining zone with the lignocellulose material being conveyed directly from the outlet of the first refining zone to a ribbon feeder feeding the second refining zone in the steam generated in the upstream stage refining zone. In high consistency refining, that is refining wherein the pulp is processed in a gaseous slurry having a solids content of 15% by weight or greater, and generally a solids content of 20% to 50% by weight, there is insufficient liquid in the pulp slurry to absorb the heat generated during the refining step without the formation of steam. Naturally, the higher the solids content, i.e. the consistency, of the pulp slurry, the greater the amount of steam generated during the refining step.